Almost all the power supplies on the market are either of a linear type or a switcher type. Linear power supplies have good regulation characteristics but a very low efficiency, large weight, large size, bad isolation from noisy lines and large temperature rises. Switching power supplies, on the other hand, have a good efficiency, low weight, small size, good isolation from line transients, low temperature rises but have usually poor regulation (especially load regulation), large ripple and noise, and large emitted interferences. In addition, the latters are usually more complex and difficult to repair. In order to have the switcher type efficiency and linear type regulation, a natural idea is to combine the two techniques to obtain a hybrid power supply.
However, simply cascading an unregulated switcher with a linear regulator would probably decrease the size and the weight of the supply by eliminating a 60 Hz transformer, but would not increase the efficiency. It could even decrease it as the efficiency of saturating core inverter is usually lower than the 60 Hz transformer efficiency.
U.S. Pat. Nos. 4,679,130 and 4,893,228 show the uses of cascading configuration of a conventional pulse width modulated switching supply with a linear regulator. However, these types of supplies should have some drawbacks as: parasitic oscillations on certain conditions induced in a feedback loop followed by another feedback loop; susceptibility to load transient even if these load variations are somewhat dampened by the linear regulator; high RF noise; and spike content easily transmitted across the linear which has usually poor rejection at high frequency.
U.S. Pat. No. 4,730,244 is an over current protection scheme implemented in the primary circuit of a conventional half bridge pulse width modulated unit. The present supply which will be described, is not a half bridge PWM unit.
Also known in the art there is the U.S. Pat. No. 4,712,169 of Manfred Albach, granted on Dec. 8, 1987, describing a circuit arrangement for forming a direct voltage from a sinusoidal input voltage. This circuit comprises among other elements, a pulse generator responsive to the input voltage to derive switching pulses for a transistor whose frequency is varied uniformly in time between a minimum frequency at the maximum value of a rectified input voltage and a maximum frequency at the minimum value. One drawback with this circuit is that the rectified input voltage must not be regulated even roughly because in that case the voltage to frequency provided in this circuit cannot detect small variations of the input voltage such as ripple signal. However, the main drawback is that it can work only with constant frequency and constant amplitude of the line, which means that no real regulation can be obtained with that circuit. Its purpose, as the inventer state it, is purely to reduce the size of the filtering choke.
An objective of the invention is to provide a power supply with better regulation than the ones known in the art, very reduced ripple and noise voltage, high efficiency, small weight, small size, easy maintenance and low cost.
Another objective of the invention is to provide a current source with better regulation than the ones known in the art, preferentially in view of realizing low weight and volume Ni-Cad battery charger.